Radio communication techniques using OFDMA (Orthogonal Frequency Division Multiple Access) schemes have conventionally been known. This OFDMA scheme is an access technique using an OFDM signal form, where terminals perform communications using slots defined by the frequency and time.
In the following description, to simplify the explanation, it is assumed that the downlink channel is in a one-frequency reuse system i.e. a system where services are provided with the same frequencies in all the cells, and that a cell is comprised of three sectors.
Further, radio communication techniques using soft-combining have conventionally been known. This soft-combining is of technique for transmitting the same data from different antennas, and combining the data to demodulate on the receiving side, and used in locations such as cell edges and sector edges with poor radio signal conditions, cases of multicast, broadcast and the like where the same data is transmitted to a plurality of terminals from different transmission locations, and the like.
FIG. 7A is a diagram illustrating a state of propagation paths to a terminal in a sector edge. In the figure, SA and SB are transmission antennas respectively of a sector A and a sector B, and MTA is an antenna of a mobile station. FIG. 7B is a diagram illustrating the propagation path between the SA and MTA, and FIG. 7C is a diagram illustrating the propagation path between the SB and MTA. When the same data is transmitted from the SA and SB, the terminal is capable of receiving a signal of combined FIGS. 7B and 7C, and the characteristics are improved. FIG. 7D shows thus combined propagation path. In this way, although different data is originally transmitted, by transmitting the same data, the reception performance is improved.
To achieve soft-combining as described above, it is considered that the same signal is generated and transmitted for a propagation path estimation signal required for OFDM signal demodulation and the data. However, the propagation path estimation signal is used not only in propagation path estimation but also in identification of a transmission antenna, and there is a case that the same pattern cannot be used.
Therefore, a conceived scheme is to transmit a propagation path estimation symbol to be only distinguished in a receiver by any method. One of methods for assuming three sectors, concurrently transmitting propagation path estimation symbols, and dividing the symbols in a reception terminal, there is the method of transmitting signals every three subcarriers in each sector. Due to OFDM, the subcarriers are orthogonal to one another, and can be separated in the receiver with reliability. At this point, the propagation path information can be obtained only in one out of three subcarriers, but it is possible to estimate the propagation path information of the other subcarriers without problems by interpolating.
FIG. 8 is a block diagram illustrating a schematic configuration of an OFDM receiver capable of being used in the above-mentioned conventional system. In FIG. 8, an OFDM symbol synchronization section 100 acquires synchronization of a received symbol. An FFT (Fast Fourier Transform) section 101 performs fast Fourier transform and transforms the signal from the time domain to the frequency domain. A propagation path 1 estimating section 102-a and propagation path 2 estimating section 102-b respectively calculate propagation path estimation values of the propagation path 1 and propagation path 2 using propagation path estimation symbols. A propagation path combining estimating section 103 estimates a combined propagation path using the propagation path estimation values output from the propagation path 1 estimating section 102-a and propagation path 2 estimating section 102-b. A propagation path compensating section 104 performs propagation path compensation using the combined propagation path estimation value output from the propagation path combining estimating section 103. A decoding section 105 decodes the data compensated for the propagation path.
Described next is the operation in performing soft-combining in OFDM reception configured as described above. The received data under goes symbol synchronization in the OFDM symbol synchronization section 100, and data corresponding to the number of points required for FFT is input to the FFT section 101. When a symbol for propagation path estimation is input to the FFT section 101, its output is input to both the propagation path 1 estimating section 102-a and propagation path 2 estimating section 102-b. Each of the propagation path estimating sections estimates a propagation path for the respective antenna based on the orthogonality (by using different subcarriers in this conventional example) of the propagation path estimation symbol. In other words, the propagation path 1 estimating section 102-a estimates propagation paths of all the subcarriers from a first transmitter that is a communicating party, and the propagation path 2 estimating section 102-b estimates propagation paths of all the subcarriers from a second transmitter that is a communicating party.
Using these signals, the propagation path combining estimating section 103 estimates a combined propagation path of paths from the first transmitter and second transmitter. The combined propagation path is obtained by vector-addition of obtained frequency response for each subcarrier.
Upon receiving a subsequent data symbol, FFT is performed similarly, and the propagation path compensating section 104 compensates the data portion for the propagation path from the combined propagation path estimation value obtained previously. By this means, the propagation path is compensated in the data concurrently transmitted from the first transmitter and second transmitter. Next, the decoding section 105 performs decoding processing and the like in response to error correcting coding processed on the transmitting side, and the transmission data can be obtained.    Non-patent Document 1: 3GPP R1-0500615 “Investigations on Inter-Sector Diversity in Evolved UTRA Downlink” NTT DoCoMo